Among its many applications, tantalum powder is generally used to produce capacitor electrodes.
Tantalum capacitor electrodes, in particular, have been a major contributor to the miniaturization of electronic circuits. Such capacitor electrodes typically are manufactured by compressing agglomerated tantalum powder to less than half of the metal's true density with an electrode lead wire to form a pellet, sintering the pellet in a furnace to form a porous body (i.e., an electrode), and then subjecting the porous body to anodization in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body. The anodized porous body is then impregnated with a cathode material, connected to a cathode lead wire and encapsulated.
The primary particle size and the agglomerated size (agglomerates are clusters of smaller primary particles), and the primary particle size and agglomerated size distributions of a powder, are important factors in the efficiency and efficacy of the subsequent sintering process by which porous bodies are made therefrom and in the electrical characteristics of the functional products, such as electrolytic capacitors, in which such porous bodies are incorporated.
In attempts to achieve a tantalum metal powder having the desirable characteristics for making capacitor electrodes and similar products, prior art powders were limited by the processes by which they were produced. Currently, for example, tantalum powders are generally produced via one of two methods: a mechanical process or a chemical process. The mechanical process includes the steps of electron beam melting of tantalum to form an ingot, hydriding the ingot, milling the hydride, and then dehydriding, crushing, and heat treating. This process generally produces powder with high purity, which is used in capacitor applications where high voltage or high reliability is required. The mechanical process suffers, however, from high production costs. In addition, tantalum powders produced by the mechanical process generally have low surface area.
The other generally utilized process for producing tantalum powder is a chemical process. Several chemical methods for producing tantalum powders suitable for use in capacitors are known in the art. U.S. Pat. No. 4,067,736, issued to Vartanian, and U.S. Pat. No. 4,149,876, issued to Rerat, disclose in detail the chemical production process involving sodium reduction of potassium fluorotantalate, K.sub.2 TaF.sub.7. A review of typical techniques is also described in the background sections of U.S. Pat. No. 4,684,399, issued to Bergman et al., and U.S. Pat. No. 5,234,491, issued to Chang.
Tantalum powders produced by chemical methods are well-suited for use in capacitors because they generally have larger surface areas than powders produced by mechanical methods. The chemical methods generally involve the chemical reduction of a tantalum compound with a reducing agent. Typical reducing agents include hydrogen and active metals such as sodium, potassium, magnesium, and calcium. Typical tantalum compounds include, but are not limited to, potassium fluorotantalate (K.sub.2 TaF.sub.7), sodium fluorotantalate (Na.sub.2 TaF.sub.7), tantalum pentachloride (TaCl.sub.5), tantalum pentafluoride (TaF.sub.5), and mixtures thereof. The most prevalent chemical process is the reduction of K.sub.2 TaF.sub.7 with liquid sodium.
The chemically reduced powder which results is referred to herein as "basic lot powder" and typically comprises agglomerates or clusters of smaller primary tantalum particles. These clusters or agglomerates are referred to herein as "basic lot agglomerates." The primary particle sizes of these basic lot agglomerates generally range from about 0.1 to about 5 microns in size. The size distribution for basic lot agglomerates for a conventional tantalum powder is shown in FIG. 1 as a comparative example. The basic lot agglomerate size distribution of basic lot powders is typically polydispersed and substantially-bimodal. As defined herein, the term "polydispersed" means a broad distribution with a wide range of values and "bimodal" means a distribution with two modes (i.e., there are two different values which are conspicuously more frequent than neighboring values).
Basic lot powder is typically heat treated, milled, or crushed, and deoxidized such as by reaction with magnesium. The resultant product, sometimes referred to herein as a "heat treated and deoxidized powder" or "finished powder" typically includes some agglomerates, which may be referred to herein as "heat treated and deoxidized agglomerates."
Products of this type may be compressed and sintered to make porous bodies, such as anodes for capacitors. Capacitor electrodes produced from such heat treated and deoxidized powders of tantalum, however, suffer from non-uniform sintering and variable porosity distributions.
The process described above is generally illustrated by the schematic block diagram included as FIG. 15 hereof.
The resultant surface area of a finished tantalum powder is an important factor in the production of capacitors. The charge capability (CV) of a tantalum (for example) capacitor (typically measured as microfarad-volts) is directly related to the total surface area of the anode after sintering and anodization. Capacitors having high surface area anodes are desirable because the greater the surface area, the greater the charge capacity of the capacitor. Greater net surface area can be achieved, of course, by increasing the quantity (grams) of powder per pellet. One way to accomplish this is by pressing larger amounts of tantalum powder to form the porous pellet before sintering. This approach is restricted, however, because there is an inherent limit to the amount of powder which can be compacted into a given pellet size. Pellets pressed with higher than usual compression ratios result in anodes having poor porosity distributions with closed and non-uniform pores. Open, uniform pores are important for the steps of anodizing and impregnating the pellet to form the cathode.
As an alternative to increasing the quantity of tantalum powder used to produce the pellet, development efforts have focused on finding tantalum powders having higher specific surface areas. By increasing the specific surface area of these powders, higher surface area anodes having higher capacitance can be achieved while using smaller amounts of the tantalum powders. These higher capacitance values are typically measured based upon the volume of pellet produced (i.e., CV/cc). As a result, by using high surface area tantalum powders, capacitor sizes can be reduced while achieving the same level of capacitance. Alternatively, a larger capacitance can be achieved for a given capacitor size.
Various tantalum powder process techniques have been practiced which attempt to maximize the production of a powder having desired small primary particle sizes and, therefore, increased surface area. For example, U.S. Pat. No. 4,149,876, issued to Rerat, concerns techniques for controlling surface area of tantalum powder product in a reduction process wherein liquid sodium is added to a molten bath of K.sub.2 TaF.sub.7 and a diluent salt.
These various other tantalum powder process techniques which produce a powder having increased surface area still result, however, in a finished tantalum powder having a size distribution which is broad and polydispersed.